Cementing operations have involved the use of plugs as a way of correctly positioning the cement when setting a liner. Some mechanisms have employed the use of pressure or vacuum to initiate plug movement downhole for proper displacement of the cement to its appropriate location for securing the liner properly. The early designs were manual operations so that when it was time to release a plug for the cementing operation, a lever was manually operated to accomplish the dropping of the plug. This created several problems because the plug-dropping head would not always be within easy access of the rig floor. Frequently, depending upon the configuration of the particular well being drilled, the dropping head could be as much as 100 ft. or more in the derrick. In order to properly actuate the plug to drop, rig personnel would have to go up on some lift mechanism to reach the manual handle. This process would have to be repeated if the plug-dropping head had facilities for dropping more than one plug. In those instances, each time another plug was to be dropped, the operator of the handle would have to be hoisted to the proper elevation for the operation. In situations involving foul weather, such as high winds or low visibility, the manual operation had numerous safety risks. Manual operations used in the past are illustrated in U.S. Pat. No. 4,854,383. In that patent, a manual valve realignment redirected the flow from bypassing the plug to directly above it so that it could be driven downhole.
Hydraulic systems involving a stationary control panel mounted on the rig floor, with the ability to remotely operate valves in conjunction with cementing plugs, have also been used in the past. Typical of such applications is U.S. Pat. No. 4,782,894. Some of the drawbacks of such systems are that for unusual applications where the plug-dropping head turned out to be a substantial distance from the rig floor, the hoses provided with the hydraulic system would not be long enough to reach the control panel meant to be mounted on the rig floor. Instead, in order to make the hoses deal with these unusual placement situations, the actual control panel itself had to be hoisted off the rig floor. This, of course, defeated the whole purpose of remote operation. Additionally, the portions of the dropping head to which the hydraulic lines were connected would necessarily have to remain stationary. This proved somewhat undesirable to operators who wanted the flexibility to continue rotation as well as up or down movements during the cementing operation. Similar such remote-control hydraulic systems are illustrated in U.S. Pat. Nos. 4,427,065; 4,671,353.
Yet other systems involve the pumping of cement on the rig floor to launch a ball or similar object, the seating of which would urge the cementing plug to drop. Typical of such a system is U.S. Pat. No. 5,095,988. U.S. Pat. No. 4,040,603 shows the general concept of a plug-release mechanism using a hydraulic circuit mounted on the rig floor. U.S. Pat. No. 5,033,113 shows generally the concept of using an infrared receiver to trigger the operation of a device such as an electric fan.
One type of previously used plug-dropping head is the model TD put out by Baker Oil Tools. This device has a plug stop to retain the plug, with a shifting sleeve which in a first position allows the flow to bypass around the plug being retained by the plug stop. Upon manual turning of a set screw, the sleeve shifts, allowing the plug stop to pivot so that the plug is released. The shifting of the sleeve also closes the bypass around the sleeve and forces pressure on top of the plug so that it is driven down into the wellbore in the cementing operation.
The apparatus of the present invention has been designed to achieve several objectives. By putting together an assembly that can be actuated by remote control from a safe location on the rig floor, the safety aspects of plug dropping have been improved. No longer will an operator be required to go up in the derrick to actuate a single or multiple levers in the context of liner cementing. Use of the apparatus and method of the present invention also eliminates numerous hydraulic hoses that need to be extended from a control panel to the final element necessary to be operated to allow the plug to drop. The plug can be dropped while the rotary table is in operation such that not only rotation but movement into and out of the wellbore is possible as the plug is being released to drop. The equipment is designed to be intrinsically safe to avoid any possibility of creation of a spark which could trigger an explosion. The equipment is compact and economically accomplishes the plug-dropping maneuver while the operator stands in a safe location on the rig floor. The actuation to drop can be accomplished on the fly while the plug-dropping head is being rotated or being moved longitudinally. Plug-dropping heads can be used in tandem and be made to respond to discrete signals. This ensures that the plugs are released in the proper order from a safe location on the rig.